Position-numeral equivalences and delayed position recognition span.

The role of equivalence class formation in delayed position recognition span performance was examined. Initially, the 3 subjects, adults with mental retardation, could not name the nine positions on the span test board. Thus, mnemonic facilitation of span performance based on position naming was not possible. Each subject, however, could name printed numerals and match these stimuli to their dictated names. Matching-to-sample training then established nine equivalence classes, each consisting of a position, a printed numeral, and the numeral name. In subsequent span tests, subject-produced numeral names led to dramatic increases in span scores. The equivalences provided precursors of a kind of verbal code that facilitated performance outside the training context.